Conventionally, many containers are known in which the level of liquid stored inside the container is detected or measured by using an ultrasonic sensor. Of these, in containers which may produce problems such as soiling of the stored liquid or corrosion of the ultrasonic sensor, when the ultrasonic sensor is installed inside the container (for example, containers for raw materials for chemical vapor-phase epitaxy which are used in the manufacture of semiconductors, etc., a battery case for an automobile, and so on), the ultrasonic sensor must be provided outside the container and the liquid level must be detected or measured from the outside.
Therefore, various technologies meeting these requirements have been proposed. For example, Japanese Patent Application Publication No. 2000-121410 discloses a liquid level detection device including: an ultrasonic sensor which abuts against an outside side surface of a container; a displacement means for displacing the ultrasonic sensor in a perpendicular direction along the outside side surface; and a liquid surface detection means for detecting a liquid level on the basis of change in the output level of a reflected wave relating to an ultrasonic wave emitted inside the container. Furthermore, Japanese Patent Application Publication No. 2000-314651 discloses a liquid level detection device including: a piezoelectric element installed on an outside bottom surface of a tank, via a matching layer; an ultrasonic wave emission means for emitting an ultrasonic wave inside the tank by causing the piezoelectric element to vibrate in a thickness direction; an ultrasonic wave receiving means which receives the ultrasonic wave reflected back by a surface of liquid stored inside the tank; and a liquid level detection means for detecting an absolute value of the level of liquid stored inside the tank, on the basis of the time from emission of the ultrasonic wave until receiving the reflected ultrasonic wave. Furthermore, Japanese Patent Application Publication No. 2009-544045 discloses an ultrasonic sensor for determining a liquid level provided with an ultrasonic wave transmitter and receiver which is located outside a housing that has a mutually connected measurement chamber and entry chamber, in the base of the housing at the periphery of the measurement chamber.